1. Field of the Invention
The present invention relates to wood meal and a method of manufacturing the same. More particularly, it relates to wood meal which is used as a filler or a coloring agent for plastic moldings, plastic sheets and plastic films, on whose surface are fixed hard particles, and which is used as an electroconductive filler for coatings for such use as a material for electromagnetic interference seal, absorbing radio wave and antistatic agent.
2. Description of the Prior Art
In order to obtain various plastic moldings, plastic sheets or plastic films (hereinafter referred to generally as plastic products) with the properties, particularly the surface properties similar to those of natural wood, and in order to obtain surface coating material or film material with such surface properties to be used on furniture and daily utensils, attempts have been made to add wood meal in the form of very fine particles to plastic materials or to coating and film materials.
Various products that are given the surface properties similar to natural wood are preferably colored on the surface in the tone similar to that of wood. It has therefore been the practice in the prior art to add a given amount of pigment together with a given amount of wood meal when plastic products are molded or surface coatings or films are formed.
Description in more detail is given below to wood meal and pigments that are added and blended in the prior art in plastic products and coating materials as a filler or coloring agent.
In the prior art, wood meal in the form of fine particles is used as a filler to prevent deformation of plastic products during molding process, to give the products desired hardness, and to improve the overall and relative moisture absorbing/releasing properties.
Attempts have been made in the prior art wood meal to improve its properties so that it would not become coagulated during handling, that it would uniformly disperse in the plastic material, and that it would prevent formation of wood vinegar gas during molding process using a molding machine.
A typical example of improved wood meal is the powder obtained by grinding the surface of particle boards that are hardened with urea base resin.
The powder obtained by grinding particle boards is used mainly because the powder thus obtained comprises very fine "granules" with relatively little fluffy fibers. In other words, a powder of very fine and smooth granules which disperse well in plastic materials for moldings can be obtained if and when appropriate means are used to grind the particle boards.
On the other hand, particle boards from which wood meal is ground lack uniformity in the wood properties, and the wood meal obtained therefrom tends to lack properties of a uniform and homogenous wood.
The grinding means such as sand paper to be employed also greatly affects the particle size of the resultant ground powder. Hard portions in the particle board formed by said hardening treatment with urea based resin become mixed in the ground powder, deteriorating the essential properties of wood meal such as moisture absorbing or releasing properties and soft texture.
It has therefore been proposed in the prior art to subject the starting material wood not only to mechanical pulverization using a cutter mill but to a pulverizer that utilizes impact, shear and friction forces such as an impeller mill in order to preserve the intrinsic properties of the material wood such as moisture absorbing and releasing properties.
A reference will now be made to pigments that are used as a coloring agent for plastic products and resin coatings.
Generally, pigments come in fine powder of inorganic or organic material, and are used as a coloring agent and an extender for plastic products and coatings.
When used as a coloring agent for plastic products, such pigments must be heat resistant insoluble in solvents, and effectively dispersed in the molding material.
When used in coating solutions, said pigments must be well dispersed in the coating solution without coagulating and without becoming settled in the solution.
When used in coatings made of powder such as powder of epoxy resin, the pigments must have a good flowability to be well blended with the resin powder.
Fine powders of metals such as Ag, Cu and Ni and of metallic oxides such as SnO.sub.2 and ZnO and carbon powders have been used as a filler for coatings to give electroconductive properties such as for electromagnetic shielding, absorbing radio waves, and antistatic agent. Such conductive fillers must be uniformly dispersed in the coating solution such as of resin.
Fine powders of inorganic material such as of calcium carbonate were used as a filler for plastic products to give dimensional stability and surface hardness required of plastic products.
These various aspects of the prior art entailed the following problems.
Firstly, the ground powder of particle boards failed to give sufficient and uniformly distributed moisture absorbing property to plastic products when the powder was blended and dispersed in the starting resin material for molding the plastic products, or when blended and dispersed in resin coatings. Rather, the powder tended to give sticky feel on the surface of the resultant product or coating.
Hard particles of thermosetting resin contained in the ground powder obtained from particle boards become exposed on the surface of the plastic products or coatings, deteriorating the intrinsic soft texture of a wood material.
Wood meal obtained by pulverizing wood material with a pulverizer that utilizes impact, shear and friction forces such as an impeller mill may retain various properties intrinsic and unique to a wood material, but its particles are fluffy and often include elongated and fibrous particles. Such wood meal is extremely inferior in its dispersing property when blended in a coating solution, nor does it disperse uniformly in various resin materials for molding plastic products or in fillers. The wood meal obtained as above easily becomes coagulated during pulverization or during storage, posing problems in handling. It also poses problems as it easily becomes coagulated in the resin material for plastic products or in the coating solutions.
When wood chips and the like are directly pulverized in a dry ball mill to obtain wood meal, the amount of resultant wood meal obtained by one cycle of pulverization is extremely small. What is more, despite prolonged pulverization, a large amount of particles with long fibers would still remain in the resultant powder, while at the same time, a large amount of over-pulverized particles is produced. The over-pulverized particles pose problems when in use as they easily become coagulated with one another.
As for the pigments used as a coloring agent, the particle size is generally very small, measuring about 1 .mu.m. Particularly in the case of an organic pigment, the primary particle size ranges between 0.1 .mu.m and 0.5 .mu.m. These extremely small particles of pigment do not disperse uniformly when blended in resin materials for molding or coating, and will not give sufficient coloring commensurate with the pigment addition. Particles of pigment easily become coagulated, and coagulated particles of pigment tend to deteriorate the surface properties of the plastic products to thereby cause uneven coloring.
Particles of pigment do not disperse uniformly in coating solutions or solvents, as mentioned earlier. They tend to become settled in the solvent or coating solution as time elapses, again failing to give sufficient coloring on the surface commensurate with the amount of pigment addition. As also said earlier, particles of pigment easily become coagulated with one another, deteriorating the surface properties of the coating to thereby cause uneven coloring.
Pigments of extremely small particles lack flowability, and when blended with a powder coating material such as epoxy resin powder, pigment particles tend to form lumps instead of being well blended.
As for the powders to be used as an electroconductive filler, the particles are selected within the size range of several microns and are directly blended and dispersed in a coating solution. Fine particles of conductive filler are also defective in that they easily become coagulated during storage, handling, addition or blending, hampering uniform dispersion.
As a result, lumps of filler particles become exposed on the surface of the coating layer, forming wavy or irregular surface, cuts or openings, and deteriorating the adhesion, heat resistance or weatherability of the coating layer.
Because of relatively heavy weight for their small particle size, particles of conductive fillers tend to become settled in the coating solution with time, resulting in uneven distribution within the solution and impairing homogeneity of the coating layer to be formed.
Electroconductivity of the coating layer itself formed by addition of the conductive filler is greatly affected by the electric coupling of the conductive particles contained. The smaller the particle size, the higher the required ratio of conductive particle addition, and the adhesion and elasticity of the coating layer deteriorate in proportion to the level of conductivity.
Calcium carbonate powder used as a filler for plastic products does not disperse well in the resin material, and often becomes settled in the solvent or coating solution when blended and dispersed therein. Properties of the resultant plastic product may vary within the product, failing to give dimensional stability and surface hardness of a desired level.